Russia’s northern offensive isn’t going well.

Ukrainian forces have captured dozens of Russian troops in the embattled town of Vovchansk, just south of the Russia-Ukraine border, dealing a major setback to Russia’s faltering northern offensive.

In heavy fighting over the weekend, Ukrainian troops surrounded as many as 400 Russians in and around a chemical plant in central Vovchansk. Thirty Russians surrendered after repeated attempts to rescue them failed, the Ukrainian Center for Defense Strategies reported.

On May 10, a force of tens of thousands of Russian troops opened a new front in Russia’s 28-month wider war on Ukraine. Attacking south across the Russia-Ukraine border, Russian battalions quickly captured a chain of lightly defended border villages—and then advanced on Vovchansk, the first big town between the border and the city of Kharkiv, 25 miles to the south.

It was there in Vovchansk that the Ukrainians chose to stand fight. Elements of several Ukrainian brigades—including the elite 82nd Air Assault Brigade—rushed north and, in several weeks of hard fighting, blocked the Russian advance just north of the Vovcha River, which threads from east to west through central Vovchansk.

The Russians rallied. The equivalent of at least two battalions with hundreds of infantry stormed the PJSC Volchansky chemical plant, on the Vovcha River’s right bank. Factories and other industrial facilities are often the locus of the fighting in Ukraine, as their big sturdy buildings can shelter troops and protect them from artillery and drones.

The Russians’ plan was apparently to capture the chemical plant and then, from there, launch a river-crossing operation in order to force their way into southern Vovchansk.

The plan failed when Ukrainian troops—perhaps from the 9th Rifle Battalion, Russian Volunteer Corps or 36th Marine Brigade—attacked west of the chemical plant and advanced several blocks to the north, cutting off the Russians in the plant from their comrades to the west.

“The Russians are surrounded here with zero chances of evacuation or reinforcements,” one Ukrainian drone operator crowed. “A bunch of dead and wounded orcs,” they added, using a slang term for Russian soldiers.